Introduction to Graphene Quantum Dots
Graphene Quantum Dots (GQDs) are quasi-zero-dimensional nanomaterials formed from a single or several graphene sheets (1–3 layers thick), which are planar in their size under 100 nanometers. GQDs also have a defined bandgap and good quantum confinement and edge effects compared to standard graphene. Such properties render GQDs very useful for electronics, optoelectronics and electromagnetics.
Synthesis of Graphene Quantum Dots
There are two types of GQD synthesis: top-down and bottom-up.
Top-Down Synthesis Methods
Top-down processes are generally the slicing of larger carbon elements (graphite, graphene, carbon nanotubes, etc.) to smaller sizes by physical or chemical transformation to produce graphene quantum dots. The benefit of this method is that the raw materials are easy to obtain and it can be mass produced, but the drawback is that the purification is difficult and the production is low.
1. Chemical Oxidation Method: Oxidising graphite using powerful oxidizing compounds (like H2SO4 and HNO3), then high-temperature treatment and solvothermal enzymatic process to transform it into graphene quantum dots.
2. Electrochemical Method: Electrochemical techniques are used to oxidize graphene or graphite into graphene quantum dots.
3. Laser Ablation Method: Graphene quantum dots are generated by laser ablation of graphite materials, producing uniformly sized dots. This method does not require strong acidic chemicals, making it environmentally friendly and controllable.
Bottom-Up Synthesis Methods
Bottom-up approaches produce graphene quantum dots by condensation, polymerisation or carbonisation of organic small molecules. It usually starts with small molecule precursors (e.g., citric acid, glucose) and tunes reaction conditions to generate quantum dots.
1. Hydrothermal Method: Organic precursor solutions are placed in a hydrothermal reactor and reacted at high temperatures to generate graphene quantum dots. This is a simple and cheap method, but depending on the carbon source can alter the nature of the quantum dots.
2. Microwave-Assisted Synthesis: Graphene quantum dots are quickly made with the uniform and efficient heating properties of microwaves. This process is straightforward, doesn't require very long reaction times, and it can be used for large production runs.
3. Sol-Gel Method: Metal precursors are placed in acid or basic solution to make a sol, the sol is polymerized into a gel and graphene quantum dots are created.
Special Synthesis Methods
There are also special synthesis methods, such as using biomass sources (e.g., straw, corn cobs) through pyrolysis or hydrothermal methods to synthesize graphene quantum dots, or directly obtaining graphene quantum dots from CVD graphene via electrochemical oxidation.
Characterization of GQDs
A variety of high quality GQDs can be produced by controlling the manufacturing process parameters such as raw materials, temperature, and reaction time.
Fig. 1 shows the HRTEM image and dots size distribution of synthesized GQDs. The high crystallinity of GQDs can be determined by the presence of its fine lattice structure, with a lattice parameter of approximately 0.21 nm. The dot size is small and uniform, with an average diameter of ca. 4.8 (±0.8) nm.
Fig. 1. (a) HRTEM image) and (b) size distribution of GQDs.
The atomic force microscopy (AFM) image of graphene quantum dots is displayed in Fig. 2, a value which means that the vast majority of graphene quantum dots are about 1-2 nm thick, that is, the quantum dots are made up of two layers of graphene. The formed graphene quantum dots are 9.6 nm in diameter and 1 nm in thickness, which makes the graphene quantum dots disk-like instead of spherical.
Fig. 2. AFM image of GQD and height profile along the line A-B of GQD
Graphene Quantum Dots (GQDs) have great PL potential thanks to their quantum confinement, edge states and surface functionalization. They are a wide range of excitation, emission at different times due to excitation, tunable emission colour, high quantum yield and excellent photostability. Table 1 Lists all the factors responsible for the photoluminescence of graphene quantum dots.
Table 1. Photoluminescent properties of GQDs
| Property | Description | Key Factors Influencing |
| Emission Wavelength | Typically ranges from blue to near-infrared (400–800 nm) | - Size and thickness of GQDs - Surface functional groups |
| Excitation Wavelength Dependence | Photoluminescence is excitation-dependent, with different excitation wavelengths producing varying emission peaks | - Degree of quantum confinement - Heteroatom doping |
| Quantum Yield (QY) | Generally ranges from 5% to 50% (depends on synthesis and functionalization) | - Synthesis method - Surface passivation |
| Luminescence Mechanism | - Quantum confinement effects - Surface/edge states | - Surface chemistry - Defects and functionalization |
| Stability | High photostability and resistance to photobleaching | - Surface functionalization - Environment (e.g., pH, solvent) |
| Solubility and Environment Effects | Photoluminescence can vary with pH, solvent polarity, and ionic strength | - Functional groups - Environmental conditions |
| Emission Tunability | Wavelength emission shifts with changes in particle size or chemical modifications | - Oxidation levels - Doping (e.g., N, S, B atoms) |
Applications of GQDs
Graphene Quantum Dots (GQDs) have wide application potential across various applications because of their unique physics and chemical characteristics. Here are some of graphene quantum dots' uses in various fields:
Biomedical Field
- Biological Imaging: Graphene quantum dots are fluorescent and toxicity free, so it is very useful for biological imaging. They might, for instance, brighten fluorescence for in situ measurements of cell metabolism and tumor photography.
- Cancer Treatment: GQDs are utilized in cancer drug delivery systems, where specific functionalization enables targeted drug release, improving therapeutic efficacy.
- Sensors: GQDs are used in biosensors to detect cancer cells, DNA, metal ions, etc., offering high sensitivity and selectivity.
Energy Storage and Conversion
- Supercapacitors: Graphene quantum dots as electrode materials improve the specific capacitance, energy density, and cycling behaviour of supercapacitors.
- Solar Cells: GQDs inside heterojunction silicon solar cells are very efficient and can boost the cell performance through electron removal by quickly getting the electrons out of the cell.
Optoelectronic Field
- Optoelectronic Devices: GQDs are extremely optoelectronic and hence they are suitable for white light LEDs, lasers, and integrated optics.
- Fluorescent Probes: GQDs are fluorescent probes for biological imaging and environmental measurements.
Catalysis and Environmental Applications
- Photocatalysis: Graphene quantum dots are catalytically excellent for photocatalytic reactions (Water splitting to make hydrogen and removal of organic pollutants).
- Environmental Remediation: GQDs are used in environmental clean up to detect heavy metal ions and organic pollutants with powerful adsorption and degradation abilities.
Other Applications
- Anti-Counterfeiting: GQDs are used to produce optical brighteners and anti-counterfeit labels for product security and traceability.
- Consumer Electronics: GQDs could be used in consumer electronics (displays, sensors, lasers etc.).
Summary
The tunable and flexible graphene quantum dots are a fresh nanotechnological step. Whether transformative in bioimaging, transformative in energy management or optoelectronic devices, GQDs are likely to be an integral part of future science and technology.
They will only become better with further exploration and exploitation of their capacities to spur innovation in various sectors.
References
- Tian, R., et al. "Solvothermal method to prepare graphene quantum dots by hydrogen peroxide." Optical Materials 60 (2016): 204-208.
- Nguyen, T. D., et al. "Fabrication and characterization of graphene quantum dots thin film for reducing cross-sectional heat transfer through smart window." Materials Research Bulletin 127 (2020): 110861.
- Cui, Y., et al. "A Review of Advances in Graphene Quantum Dots: From Preparation and Modification Methods to Application." C 10.1 (2024): 7.
